1. Field of the Invention
The present invention relates to ball valves and, in particular, to ball valves for use with fluid mediums having large variations in temperature.
2. Description of the Invention Background
Ball valves are well known in the fluid control industry and are generally used to control the flow of a myriad of liquids ranging from water to unrefined oil. Because a ball valve may be fully opened or closed by a relatively short rotational turn of its stem, it is well adapted to be actuated by a variety of rotary actuators and, therefore, is frequently chosen over gate valves in applications where process automation is desired.
Conventional ball valves, however, are notorious for having inherent leakage problems, and thus, their use in various applications where no leakage can be tolerated has generally been avoided. In the past, small valve leaks were generally expected and accepted when using ball valves. Today however, due to various governmental regulations and environmental concerns, many industries cannot tolerate any leakage in their piping systems. For example, manufacturers handling hazardous chemicals can not safely use valves that permit the chemicals to leak into the plant environment.
This leakage problem is generally amplified when the ball valve is used in systems handling various fluid mediums having significant variations in temperature. For example, in the dairy and food processing industries, steam is admitted into the piping system to clean and disinfect the system. The steam has a much higher temperature than the food or milk product normally transported therein. As such, the valve experiences a significant change in temperature which causes the seals and the valve body to expand and contract. It has been observed that the seals do not expand and contract at the same rate as the valve body. The difference in the rates of expansion between the two material's when subjected to temperature extremes produces a gap therebetween. The fluid medium is then free to exit the system through these gaps in the valve's seal structure. In addition, harmful bacteria may also invade the piping system through the gaps to contaminate the entire system.
The leakage problems encountered in conventional ball valves can be attributed to their basic design and sealing structure. A prior art ball valve, designated as 210, is illustrated in FIG. 1. Ball valve 210 generally consists of a valve body 212 and end fittings 214 and 216. End fittings 214 and 216 have passages 218 and 220 therein and are adapted to be connected to two corresponding mounting flanges provided in the pipeline (not shown).
A valve chamber 222 is provided within valve body 212 which rotatably houses ball member 224. Ball member 224 has an axial port 225 therethrough that substantially corresponds with passages 218 and 220 in end fittings 214 and 216. A valve stem 232 extends through a bore 234 provided in valve body 212 and is non-rotatably received in a rectangular socket 230 located in the top of ball member 224. The valve 210 is operated in a well known manner by rotating valve stem 232 by a handle 250 or a rotary actuator (not shown) which in turn rotates ball member 224 within valve chamber 222. By rotating ball member 224 so that port 225 aligns with passages 218 and 220, the liquid medium is permitted to pass therethrough. To stop the liquid flow, ball member 224 is rotated so that port 225 is at a right angle with respect to passages 218 and 220.
To inhibit liquid leakage through valve 210, a number of seals are provided. In particular, valve seats 226 and 228 are positioned on opposite sides of ball member 224 to seal the joints between ball member 224 and end fittings 214 and 216. Ball member 224 is mounted within valve chamber 222 in sliding engagement with valve seats 226 and 228. To inhibit fluid leakage along the valve stem 232, a stem seal 236, fabricated from material such as a tetrafluoroethene, available from E. I. duPont deNemours & Co. under the trademark Teflon, is provided around valve stem 232 within bore 234. Teflon can be extruded at a temperature of approximately 400.degree. F. and has a useful temperature range of approximately -100.degree. F. to +480.degree. F. At temperatures greater than 480.degree. F., the Teflon seals begin to degrade.
A metal gland ring 238 is slidably received on valve stem 232 between stem seal 236 and flange 239 located on valve stem 232. Gland ring 238 is adapted to engage and compress stem seal 236. A gland nut 240, having a smooth bore 242 therethrough, is threadably received in threaded bore 235. Valve stem 232 extends through smooth bore 242 and is free to rotate therein. As leakage is encountered along valve stem 232, gland nut 240 is further threaded into threaded bore 235 causing gland ring 238 to compress stem seal 236. As stem seal 236 is compressed, it is forced against valve stem 232, thereby inhibiting the passage of the liquid along valve stem 232. This action, while inhibiting fluid leakage past valve stem 232, increases the amount of torque required to actuate the valve due to the amount of force pressing against the valve stem 232. Also, opening and closing valve 210 tends to cause gland nut 240 to back out of threaded bore 235 thereby increasing the likelihood of fluid leakage.
To inhibit the passage of fluid between valve body 212 and end fittings 214 and 216, body gaskets 244 and 246 are provided therebetween. Gaskets 244 and 246 are fabricated from Teflon material and have two relatively flat bearing surfaces for mating against the respective end fitting 214 or 216 and valve body 212.
It is a well known physical principle that all solids expand in volume when their temperatures are increased. The change in any linear dimension of the solid, such as its length, width or thickness, is called a linear expansion. The rate of linear expansion of a particular material is based primarily upon the material's atomic structure and can be predicted by mathematical equations. For example, it is well known in the art that Teflon expands at a rate approximately eight times greater than the rate at which stainless steel expands when exposed to the same increase in temperature. Therefore, when the ball valve 210 is exposed to a high temperature medium such as steam, the linear expansion of Teflon gaskets 244 and 246 will be approximately eight times greater than the linear expansion of valve body 212 and end fittings 214 and 216. Conversely, when a cooler liquid is thereafter introduced into valve 210, Teflon gaskets 244 and 246 along with valve body 212 and end fittings 214 and 216 contract in much the same manner. However, it is well known that for temperature changes in the range of 480.degree. F. or greater, Teflon does not return to its original shape. Therefore, as gaskets 244 and 246 are heated and then cooled at temperature swings of 200.degree. F. or greater, liquid is permitted to pass between end fittings 214 and 216 and valve body 212 due to the failure of gaskets 244 and 246 to assume their original shape.
Thus, there is a need for a ball valve with an improved sealing structure that can be used in systems handling liquids having temperature swings of 200.degree. F. or greater.